And how would that be different from my bass? Also, how to monitor synth on stage, and why would that be different from bass?
Because you are asking more of a stayed rig than they are designed for. They aren't designed to produce the fundamental of a low F#. If you out a high pass filter between the synth amd stayed rig, you could get by hearing the first or second harmonics. But you would not want to simply run a synth (being deployed for the specific purpose of producing very low notes) into a traditional bass rig. If you need stage monitoring, your best bet would literally be a PA stack. In other words, you would need a top and a sub, similar to what a small band would use as their mains. For years, drum monitors on larger stages looked like this.
You seem to be getting a little bit of a chip on your shoulder toward people who are trying to help you.
The simple fact if the matter is what you're wanting to do is impractical given today's tech.
Want a simple solution? Go in-ears. Good in-ear buds would allow you to put whatever you want in your monitors in a tiny package.
But, to stick to the way you want to do things, you will need a bass rig and bass, a synth, and a PA top cab and sub for your synth stage monitor. That seems like a lot to haul around. Maybe if you got a small mixer and a preamp for bass you could send both your bass and the synth through your PA stick stage monitor. But I bet you would then get into volume wars on stage quickly. Never mind that producing those very low notes and having them blast all over the stage (rather than simply out of the PA mains) would've for a miserable, muddy, stage volume situation.
But what do I know? I only have 35 years of experience playing bass live and running sound. You might be "different", or not "typical", but the laws of physics still apply to you.
Either way, I'm tapping out of this discussion. Best of luck with finding a solution.